Semiconductor leadframes serve as substrates for the manufacture of certain semiconductor packages. Conventionally, leadframes are made from iron alloys. However, with an increasing demand for higher performance miniaturized packages, more reactive metals, in particular copper alloy leadframes are finding increasing applications in semiconductor packages. These leadframes are found to be more attractive than iron alloy leadframes due to factors such as better heat dissipation, ease of processing and cost. On the other hand, the disadvantage of copper alloy is that it is prone to oxidation (ie. it reacts with oxygen to produce copper oxide) when exposed to oxygen in the air at high temperatures. Such oxidation results in oxygen forming weak bonds with the atoms at the leadframe surface, and a layer of brittle and/or poorly adhering oxides. Thus, oxidation introduces reliability problems for microelectronic packages.
The problem of oxidation is particularly acute during wire-bonding in a typical semiconductor packaging process, wherein conductive bonding wires are bonded to contact surfaces on a semiconductor die and a leadframe to establish electrical connections therebetween. This bonding is commonly done by using an ultrasonic transducer to generate mechanical vibration energy with an external pressure force to bind the wire to the die and leadframe surfaces. However, heat generated during the process may oxidize the surface of the leadframe, leading to non-stick or unreliability of the bond. Oxidation during the wire-bonding process should thus be arrested or reduced.
A typical wire bonder uses a window clamp, which is usually rectangular in design, to clamp a leadframe securely to a top plate. An industry practice for protecting leadframes from oxidation is to introduce large amounts of a relatively inert gas, usually nitrogen gas, to the leadframe. Various apparatus have been used to do this. In a typical apparatus, an area of the leadframe is covered by the body of the clamp and is thus relatively well-protected from oxidation, leaving an opening inside the body of the window clamp as a bonding area. The bonding area is exposed to the atmosphere, and is most vulnerable to oxidation.
One method of introducing nitrogen gas is to locate one or more nozzles next to the bonding area to blow nitrogen gas into the bonding area. (FIG. 1) The nitrogen in the environment around the bonding area would tend to inhibit oxidation reaction of the leadframe at the high bonding temperatures. However, the use of nozzles creates a negative pressure around the region of the nozzle opening, sucking air towards the nozzle opening. After several minutes, the effectiveness of the nozzle will be diminished due to the oxygen drawn in from the atmosphere mixing with the nitrogen gas discharge.
An example of such a method is disclosed in U.S. Pat. No. 5,265,788 entitled, “Bonding Machine with Oxidization Preventive Means”. The oxidation preventive assembly described is made up of two pipes installed on a bonding stage. The two pipes are formed with gas discharge holes and their terminal ends are closed by a block to prevent a back-flow of the gas supplied into the pipes. Thus, a uniform gas atmosphere is created around the workpiece which is placed on the bonding stage, preventing oxidation of the workpiece. Nevertheless, due to the negative pressure around the gas discharge holes as explained above, oxygen will consequently be drawn in from the atmosphere so that the effectiveness of the assembly is reduced.
Another method of introducing nitrogen gas is to have one or more gas blower outlets in a top plate on which the leadframe rests for the gas to be discharged into the bonding area. (FIG. 2) This method has a disadvantage in that it is difficult for a manufacturer to manufacture a top plate with many small holes as outlets for the nitrogen gas, bearing in mind that the surface of the top plate has to be substantially even to allow wire-bonding to be effectively performed on it.
This design also has the problem of negative pressure being created around the outlets of the top plate. After a while, the negative pressure causes oxygen in the air to be drawn to the openings and mixed with nitrogen gas, reducing its effectiveness. Furthermore, it should be appreciated that such a method only works where the surface of the leadframe itself has through-holes to allow gas to enter the bonding area inside the wire clamp. If there are no such through-holes, the method is not effective.
A third method is to use a movable cover together with the first method and/or the second method (FIG. 3), its purpose being to prevent as far as possible nitrogen gas from escaping from the bonding area. The movable cover has a through-hole to allow a capillary of a bonding member to extend into the bonding area. However, the addition of an additional part to the wire bonder (specifically, the bond head of the wire bonder) affects the bonding performance of the machine. The cover will also block an operator's view of the leadframe as it is being bonded, and makes the bond area inaccessible when, for example, a bond wire breaks. The cover size is also too large when there is a large bond area. Moreover, there is a risk of the wire clamp hitting the moving cover when the wire clamp is moved up to release a leadframe.